This invention relates generally to nuclear magnetic resonance (NMR) or magnetic resonance (MR) imaging, and more particularly the invention relates to the selective projection imaging of moving material by magnetic gradient manipulation.
Techniques are known for magnetic resonance angiography in which blood flow is imaged. U.S. Pat. No. 4,528,985 utilizes a temporal subtraction technique in which image data at two different time intervals is obtained and subtracted so that data for static material cancels and data for moving material (e.g., blood) provides a residual image.
U.S. Pat. No. 4,516,582 excites nuclear spins in a thin slab and employs a gradient field to dephase excited spins of static material. After a time interval in which excited nuclear spins of blood flow from the thin slab, the nuclear spins and the slabs are again excited. Due to the dephasing to the static spins, substantially all of the subsequent resulting NMR signal will come from the excitation of spins of blood flow which moves into the slab during the time interval.
U.S. Pat. No. 4,647,857
for FLOW MEASUREMENT USING NUCLEAR MAGNETIC RESONANCE utilizes spin echo techniques to eliminate the effects of static nuclear spins whereby a residual signal from dynamic or moving nuclear spins is obtained.
Disclosed in copending application Ser. No. 894,318, filed Aug. 7, 1986, now U.S. Pat. No. 4,718,424 is a method of imaging blood flow utilizing the selective effects of magnetic gradient field waveform moments on static material and on flowing material. By varying the polarity and duration of a magnetic field gradient in which material is positioned, the magnitude or phase of nuclear spins signals and FIDs can be varied depending on the motion of the material in a direction aligned with the magnetic field gradient.